Conventionally, a method of a technology to reduce damage of a GaN layer, which is made from a group-III element nitride, is proposed in, for example, Patent Document JP-B-4479222 (corresponding to US 2004/0192043 A1). A compound semiconductor layer includes a first compound semiconductor layer and a second compound semiconductor layer. The second compound semiconductor layer is layered on the first compound semiconductor layer, and the first compound semiconductor layer includes nitrogen. The second compound semiconductor layer is dry etched, so that the first compound semiconductor layer is partially exposed. After the dry etching process, a damage layer caused by the dry etching process is removed by heat process in a nitrogen atmosphere, and then an etching process is performed by a nitrogen plasma, so that the damage caused by a nitrogen vacancy in the first compound semiconductor layer is recovered.
In addition, another method to perform a plasma process with a substrate including a compound semiconductor is known. In this method, a halogen gas is included in a process gas. The substrate is heated to be equal to or more than a boiling point of a compound including substrate material and a halogen element in the halogen gas, and a surface of the substrate is processed.
The inventors of the present disclosure have found the following difficulties.
In the method described in Patent Document JP-B-4479222, after the dry etching is performed, in order to reduce the damage caused by the dry etching process, the heat process and a post process using the nitrogen plasma need to be added. Therefore, a manufacturing process may become more complicated, and a manufacturing cost may be increased.
In addition, when a plasma process, in which the substrate is heated equal to or more than a boiling point of a compound including a halogen gas and a substrate material, is used, the boiling point is high, and therefore a resist process is not applicable. As a result, a manufacturing process may be complicated. For example, when a chloride gas is used as the halogen gas, the boiling point of GaCl3, which is a reaction product with the GaN layer, is 201 degree Celsius. It is high enough that a resist is carbonized. Thus, it is impossible to use a simple resist in the dry etching process. Therefore, it is necessary to accumulate a hard mask such as a SiO2 layer or the like on the GaN layer, and to pattern the SiO2 layer or the like.